Optical communications refers to the medium and the technology associated with the transmission of information as light pulses. Many applications utilize an optical fiber network to establish optical communications between network locations. In order to enable optical communication and the flow of optical signals between network locations, various interconnections must be established between different optical fibers.
Optical fiber cable consists of a plurality of optical fibers surrounded by protective sheath. Each individual optical fiber ("fiber") consists of a small diameter core of low-loss material such as glass or plastic surrounded by a protective cladding that has a slightly lower index of refraction than the core. Light, as it passes from a medium of higher index of refraction to one of lower index of refraction, is bent away from the normal to the interface between the two media. At a critical angle of incidence, transmitted light is totally reflected within the medium having the higher index of refraction. Building on these basic rules of physics, optical fibers are designed and made such that there is essentially total reflection of light as it propagates through an optical fiber core. Thus, the core is able to guide light pulses with small attenuation of transmitted light pulses and low signal loss.
In many cases of signal transmission via optical media, a key transmission parameter is signal loss per distance transmitted. Due to the fragile nature of the core of an optical fiber, there is a need to protect an optical fiber from external sources of stress, such as bending, pressure and strain, that can damage the fiber and may also increase signal loss. For example, an optical fiber should not be bent sharply anywhere along its path. In addition to the possibility of breakage or fracture, if an optical fiber is bent past a critical angle, portions of transmitted light pulses will not be reflected within the core of the optical fiber and will no longer traverse the optical fiber. These attenuated portions of light pulses result in signal loss and thus, degradation of signal quality.
Referring to FIG. 1a, there is shown a simple ray model of light pulse transmission on a straight optical fiber. The optical fiber 100, shown in longitudinal cross section, has an optical core 102 which is surrounded by a cladding 104 and has a critical angle .theta..sub.c. FIG. 1b shows a simple ray model of light pulse transmission in a bent optical fiber. As illustrated, when the bend of the optical fiber 100 is such as to cause a light ray to strike the boundary of the core 102 and cladding 104 at an angle greater than the critical angle .theta..sub.c --the angular excess, as shown in the inset, being labeled .theta..sub.bend --the light ray leaks out of the optical fiber core. Further, while lower order mode light rays are not likely to leak out of the optical fiber core, they may be transformed into higher order mode light rays and may leak out at a subsequent bend in the optical fiber. Accordingly, it is necessary that an optical fiber be routed so that bends in the optical fiber be of a sufficient radius to substantially avoid occurrence of such an extra critical angle, and the associated light leakage.
The minimum bend radius characterizes the radius below which an optical fiber should not be bent to avoid light ray leakage. Typically, the minimum bend radius varies with fiber design. Bending an optical fiber with a radius smaller than the minimum bend radius may result in increased signal attenuation and/or a broken optical fiber.
Ordinarily, a unique optical fiber routing will be required to transmit light pulses between network locations. Over this unique route, light pulses may be propagated across several different optical fibers. At each transition from one fiber to another, individual optical fibers may be joined together, thereby enabling light pulses to be carried from/between a first fiber and a second fiber. Once made, a connection must be held securely in place to prevent a loss of transmission quality. Transmission via optical fiber also requires repeating (i.e., amplifying) the transmitted optical signal at distance intervals. Consequently, optical fiber connections also must be made at the distance intervals where such signal repeater equipment is needed.
It may be necessary to bend optical fibers around comers and other obstacles in order to route the optical fiber to/from optical fiber network equipment and accomplish the required connections. While performing such activity, stresses on the optical fiber must be limited. Moreover, connections of optical fibers need to be isolated and protected from environmental degradation, strain, and torque in order to maintain the proper alignment between connected optical fibers and to avoid undesirable signal attenuation.
Previously, closures have been designed to protect connections of copper wire. A closure typically houses a cable interconnect frame and provides mounting surfaces for electronics and apertures for cabling to pass to/from the enclosed chamber of the closure. An articulated door is usually provided for access to the enclosed interconnect frame and electronics compartment. However, optical fiber closures present a host of different complexities revolving around bend limiting and the minimum bend radius as described above.
Existing closure architecture does not always integrate installed optical fibers well with regard to the maintenance of the minimum bend radius for the fibers. Further, such closures provide no or limited means to control, organize, or stow optical fibers and fiber slack routed in the interior or along the exterior of the closure. For instance, provisions are not made for maintaining the minimum bend radius of fibers entering/exiting a closure resulting in fibers being jostled while accessing the closure. Consequently, special care is required in order to access (i.e., open and close) the closure without exceeding fiber bend limits or breaking optical fibers, which action could cause a system malfunction. Thus, a technician faces many logistical problems during maintenance of optical fibers within such a closure, adding significant complexity and time to that required to remove, repair and/or replace fibers in a closure.